In history class, the girl to boy ratio is $4$ to $3$. If there are a total of $21$ students, how many girls are there?
Explanation: A ratio of $4$ girls to $3$ boys means that a set of $7$ students will have $4$ girls and $3$ boys. A class of $21$ students has $3$ sets of $7$ students. Because we know that there are $4$ girls in each set of $7$ students, the class must have $3$ groups of $4$ girls each. There is a total of $12$ girls in history class.